Is there enough water?
About 71% of Earth’s surface is covered by water, but almost all of it is seawater. Fresh water is only about 2.5% of the total, and most of that is locked in glaciers or deep underground, so very little is easy for people to use. Understanding water, cleaning it, and saving it all take chemistry.
What water is made of: electrolysis
You’ve already seen water split by electricity in lessons 1.1.1 and 1.6.1. Now look at the experiment closely. A little sodium sulfate or sodium hydroxide is added so the water conducts electricity, and a direct-current power supply is switched on.
Observations: bubbles form at both electrodes. After a while, the gas at the negative electrode has about twice the volume of the gas at the positive electrode.
Tests: the gas at the positive electrode relights a glowing splint (and makes a burning splint burn brighter), so it is oxygen. The gas at the negative electrode burns with a pale blue flame, so it is hydrogen. A handy way to remember: hydrogen at the negative side, and twice as much hydrogen as oxygen.
Conclusion: elements don’t change in a chemical reaction. Splitting water produced hydrogen and oxygen, so water is made of the elements hydrogen and oxygen.
Hydrogen is the lightest gas, barely dissolves in water, and burns in air to form water, which also confirms what water is made of:
Hydrogen mixed with air can explode if it meets a flame, so always test its purity before lighting it.
Treating drinking water
Natural water contains mud and other insoluble matter, some dissolved substances, and possibly germs. A typical water treatment plant works like this: take in water → add a coagulant (such as alum) → let it settle → filter → pass it through activated carbon → disinfect → send it to homes.
| Step | What it does |
|---|---|
| Settling (with a coagulant) | Alum forms a sticky gel that grabs suspended particles so they sink |
| Filtering | Removes insoluble particles (lesson 1.2.7) |
| Adsorption | Porous activated carbon traps substances that cause color and odor |
| Disinfection | Kills bacteria and viruses (a chemical change, often using chlorine) |
| Distillation | Produces almost pure water; the most thorough method |
Settling, filtering, and adsorption can’t remove dissolved substances, so the water after these steps is still a mixture.
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Hard water and soft water
Hard water硬水 contains a lot of dissolved calcium and magnesium compounds; soft water软水 contains little or none.
- Testing: add soapy water and shake. Lots of suds and little scum means soft water; few suds and lots of scum means hard water.
- Problems: hard water wastes soap and leaves clothes dingy, and it forms scale in kettles, pipes, and boilers, wasting fuel.
- Softening: at home, boiling reduces hardness, and many homes in hard-water areas use ion-exchange water softeners. In the lab, water is softened by distillation.
Finally, fresh water is precious. Save it (reuse water, fix leaks, use water-saving fixtures) and keep it clean (treat industrial and household wastewater before release, and use fertilizers and pesticides carefully).
Example 1: Reading electrolysis data
During electrolysis, 10 mL of gas is collected at the positive electrode. How much gas should collect at the negative electrode? What does the experiment show?
- Identify the gases. Oxygen at the positive electrode, hydrogen at the negative.
- Use the volume ratio. Hydrogen and oxygen form in about a 2 : 1 volume ratio, so about 20 mL of hydrogen collects.
- Conclusion. Water breaks down into hydrogen and oxygen, and elements don’t change in reactions, so water is made of hydrogen and oxygen.
- A detail. Oxygen is a little more soluble in water than hydrogen, so real measurements often come out slightly above 2 : 1.
Example 2: Is your tap water hard or soft?
- Sample. Fill two test tubes with equal amounts of tap water and distilled water.
- Add soap. Add the same number of drops of soapy water to each and shake.
- Compare. Distilled water is soft and makes lots of suds. If the tap water makes noticeably fewer suds and more scum, it’s hard.
- Treat. Boiling turns some of the dissolved calcium and magnesium compounds into scale, which reduces the hardness.
- Adding alum so suspended particles clump together and sink
Alum forms a gel that grabs suspended particles so they settle out.
- Removing the mud that has settled to the bottom
Filtering separates insoluble solids from water.
- Removing color and bad odors
Porous activated carbon adsorbs them.
- Killing bacteria in tap water
Disinfection kills germs; it's a chemical change.
- Getting the purest possible water in the lab
Distillation removes almost all impurities.
- Turning hard water into soft water in the lab
Distillation leaves dissolved calcium and magnesium compounds behind. At home, boiling helps.
- The carbon layer in a pitcher filter that removes odors
That's activated carbon adsorbing them, a physical process.
Common mistakes
Mistake 1: “Water is made of hydrogen gas and oxygen gas.” Water is a pure substance and contains no hydrogen gas or oxygen gas. Say instead: water is made of the elements hydrogen and oxygen, and each water molecule contains 2 hydrogen atoms and 1 oxygen atom.
Mistake 2: “Hydrogen forms at the positive electrode.” Hydrogen forms at the negative electrode, with about twice the volume of the oxygen at the positive electrode.
Mistake 3: “Filtered water is pure, and activated carbon softens hard water.” Filtering and adsorption remove insoluble matter, color, and odors, but not dissolved calcium and magnesium compounds, so they don’t soften water.
Quick check
What does the electrolysis of water show?
Elements don't change in a reaction, and the products contain only hydrogen and oxygen, so water is made of those two elements. A and C wrongly put the gases inside water, and D should say one water molecule.
What's a common way to reduce water hardness at home?
Boiling turns some dissolved calcium and magnesium compounds into scale. Filtering, activated carbon, and alum don't remove dissolved calcium and magnesium compounds.
Summary
- Electrolysis: hydrogen at the negative electrode, oxygen at the positive, in a 2 : 1 volume ratio. shows water is made of hydrogen and oxygen. Test hydrogen’s purity before lighting it.
- Water treatment: settling, filtering, adsorption, and disinfection; distillation gives the purest water. Water after filtering and adsorption is still a mixture.
- Hard water contains more dissolved calcium and magnesium compounds. Test with soap; soften by boiling at home or distilling in the lab.
- Save water and keep it clean.
On to AP (for later)
Electrolysis of water takes place in an electrolytic cell (AP topic 9.7): electricity drives a redox reaction that wouldn’t happen on its own. Water is reduced to hydrogen at the negative electrode (the cathode) and oxidized to oxygen at the positive electrode (the anode). At the same temperature and pressure, gas volumes are proportional to moles (topic 3.4), so the 2 : 1 volume ratio matches the coefficients in the equation. The and ions in hard water forming scum with soap are a real-life precipitation reaction from Unit 4.